How come United only flies to Heathrow in the UK while AA is serving LHR, Gatwick, Birmingham, Manchester, Glasgow. I know that UA is unlikely to start any new routes now due to September 11th but before this it only flew into 1 UK destination while AA flies to all major UK cities. Isn't there sufficient traffic between say, IAD and LGW or ORD LGW/MAN.

Right now, there is a simple answer: Star Alliance partner British Midland. They pretty much cover all of the UK very effectively (and I believe are covering the Manchester market with flights from IAD and ORD).

Although the reason why they don't serve cities besides London probably has to do with the strength of their alliance partner bmi, that still leaves Gatwick, which bmi has little if any presence at. Personally, I think that UA doesn't serve Gatwick because it would simply not be economically feasible to open up a station at Gatwick which would only serve flights from one city (Denver). UA simply has no need to serve LGW from any other city not covered in the Bermuda II agreement. Plus, the fact that there aren't many connection opportunities at Gatwick (again, lack of bmi presence, as well as no onward destinations such as AMS and DEL/HKG that UA had in the past) contributed to UA's decision not to start flights to Gatwick.